Figure 8-5.The ac welding cycle.Figure 8-4.Effects of polarity on the weld.also turn the main welding current on and off at the sametime. This not only allows the operator to start and stopwithout leaving the work but also to adjust the currentwhile welding.Most of these welding machines can produce bothac and dc current. The choice of ac or dc depends on thewelding characteristics required.DIRECT CURRENT. As you learned in chapter7, a direct-current welding circuit maybe either straightor reverse polarity. When the machine is set on straightpolarity, the electrons flow from the electrode to theplate, concentrating most of the heat on the work Withreverse polarity, the flow of electrons is from the plateto the electrode, thus causing a greater concentration ofheat at the electrode. Because of this intense heat, theelectrode tends to melt off; therefore, direct-currentreverse polarity (DCRP) requires a larger diameter elec-trode than direct-current straight polarity (DCSP).The effects of polarity on the weld are shown infigure 8-4. Notice that DCSP produces a narrow, deepweld. Since the heat is concentrated on the work, thewelding process is more rapid and there is less distortionof the base metal. Overall, straight polarity is preferredover reverse polarity because you can achieve betterwelds.DCRP forms a wide and shallow weld and is rarelyused in the GTAW process. The exception to this is whenit is used to weld sections of aluminum or magnesium.DCRP has excellent cleaning power that results from theFigure 8-6.ACHF combines the desired cleaning action ofDCRP with the good penetration of DCSP.action of positive-charged gas ions. When these gas ionsstrike the metal, they pierce the oxide film and form apath for the welding current to follow. This same clean-ing action occurs in the reverse polarity half of analternating-current welding cycle.ALTERNATING CURRENT. AS shown in fig-ure 8-5, ac welding is actually a combination of DCSPand DCRP; however, the electrical characteristics of theoxides on the metal often prevent the current fromflowing smoothly in the reverse polarity half of thecycle. This partial or complete stoppage of current flow(rectification) causes the arc to be unstable and some-times go out. Ac welding machines were developed witha high-frequency current flow unit to prevent this recti-fication. The high-frequency current pierces the oxidefilm and forms a path for the welding current to follow.The effects of alternating current high-frequency(ACHF) are shown in figure 8-6. Notice that ACHFoffers both the advantages of DCRP and DCSP. ACHFis excellent for welding aluminum.8-3